Lyme disease, the most prevalent arthropod-borne disease in the United States, is caused by infection with the spirochete, Borrelia burgdorferi. Despite intensive study in recent years little is known regarding the pathogenesis of infection at the molecular level. In recent studies, the investigators have shown that B. burgdorferi sensu stricto isolates obtained from patients with early Lyme disease can be distinguished on the basis of several genotypic parameters and that the distribution of these genotypes is significantly different in isolates cultured from the skin or blood of patients with early Lyme disease. This suggests that different genotypes of B. burgdorferi sensu stricto vary in their potential for bloodstream dissemination. The investigators hypothesize that differences in potential for hematogenous spread are the result of variations in gene expression among different genotypes and propose to identify the factors underlying these differences by a combination of genomic and proteomic approaches. The expression of genes identified by such analysis will then be monitored in Lyme disease patients. The specific aims of the project are: 1) to assess differences in gene expression among the various genotypes by high resolution analysis of total B. burgdorferi cellular protein. Those proteins which are unique to specific genotypes or which show significant variations in expression (under specific growth conditions) will be identified by microsequencing and comparison to sequences in the B. burgdorferi genome database; 2) to analyze expression for a selected group of genes by high density membrane arrays techniques and by RT-PCR of isolates cultured under a variety of physiological conditions; 3) to analyze the expression of identified, putative virulence determinants by RT-PCR analysis directly in skin biopsy tissue and blood of Lyme disease patients and in tissues of experimentally infected mice. The combination of functional genomics and in vivo approaches proposed should provide for the comprehensive assessment of the molecular basis of pathogenesis for different genotypic variants of B. burgdorferi sensu stricto and should result in identification of determinants required for spirochete dissemination. This should provide new insights into the natural history of B. burgdorferi infection in humans, which, in turn, may have implications for prevention, treatment, and diagnosis of Lyme disease.